1. Field of the Invention
This invention relates to a circuit for accurately converting analog video signals output from a video graphic array (hereinafter referred as VGA) card into digital video signals, and more particularly to a circuit for converting analog video signals output from a VGA card into digital video signals served to a liquid crystal display (hereinafter referred as LCD) panel.
2. Description of the Prior Art
Since cathode ray tube (hereinafter referred as CRT) color monitors adopt analog signals as input, commercial VGA cards are all designed to be adapted to this manner. Besides, commercial VGA cards have various resolutions, and thus have various output frequencies. For CRT multi-frequency color monitors, there are various designs and applications.
Since the volume of an LCD panel (or referred as an LCD monitor) is much smaller than that of a CRT monitor, with the progress in LCD technologies, LCD monitors will gradually take the place of conventional CRT monitors in the future. Accordingly, it is significant to display the analog video signals output from VGA cards on the LCD monitors.
In order to display the analog video signals output from VGA cards on the LCD monitors, there are many problems to be solved. First, commercial LCD monitors do not have the functions of multi-frequency and multi-resolution. For example, it is impossible to acquire an 800*600 resolution on an LCD monitor having a 1024*768 resolution. Accordingly, when an LCD monitor is used to display a video signal, if different resolutions are required, the video signal must be processed to be converted into signals which can be served to the LCD monitor.
Second, since the LCD monitor is operated in the digital manner, the analog video signals output from a VGA card must be converted into digital signals by an analog-to-digital converter (hereinafter referred as A/D converter) to be displayed on the LCD monitor. A video signal is composed of a majority of pixels, each of which is to be precisely converted into a digital signal, which is involved with synchronization and phase alignment.
Furthermore, the way how the video signals can be displayed on the correct positions of an LCD monitor must be solved as well.
Commercial analog LCD multi-frequency color monitors have following disadvantages: (1) To overcome the problems of multi-frequency and different resolutions, auto-detecting or manual tuning has to be utilized; (2) Manual tuning has to be performed such that video signals are displayed at adequate positions; and (3) Manual tuning is indispensable to stablize the displayed image, however, most users cannot do this.
On the other hand, although some products are asserted to be able to perform auto-tuning, the associated accuracy cannot satisfy our requirements and some special image patterns are needed to efficiently perform the auto-tuning. However, such special image patterns are not usually obtainable for practical computer applications.
FIG. 1 illustrates a block diagram for converting analog video signals into digital video signals in a conventional analog LCD multi-frequency color monitor. FIG. 2 shows an ideal timing for converting an analog signal into digital signals accurately, wherein each active edge (trailing edge) of the clock signal A/D CLOCK corresponds to the middle of a pixel. Therefore, each pixel can be accurately converted into a digital signal.
In order to achieve the above object, the following conditions must be satisfied: (1) The frequency of the clock signal A/D CLOCK must be the same as that of the pixel frequency of the original video signal, no error is allowed. This frequency is determined by a phase-locked loop of the VGA card, which cannot be predicted in advance by the user; and (2) Under the situation that the frequency is correct, the phase is needed to be accurately aligned such that correct digital signals can be derived.
FIG. 3 illustrates the situation that the frequency of the clock signal A/D CLOCK is not the same as the pixel frequency of the original video signal. Since it is impossible to make each active edge of the A/D CLOCK corresponds to the middle of a pixel, sampling results may be wrong or ambiguous, which results in bad images on the LCD monitor.
To search for the desired pixel frequency accurately, a digital signal processor can be utilized, whose disadvantage is that since the pixel frequency is quite high (about 10 MHz.about.100 MHz), even ordinary digital signal processor cannot be realized easily and the cost is very expensive.